In view of the current situation that the basic accuracy is generally adopted as the final technical indicator of the instrument in the process of measuring and using precise digital measuring instrument, the paper has analysed the principle of precise digital measuring instrument and the factors that affect the measurement accuracy, elaborated the calculation method of the maximum permissible errors of the digital measuring instrument in detail, and proposed the “point-by-point calculation scheme” of capacitance parameter by taking E4980A as a typical model. and finally found the calculation result of the same measurement point is ±11.95% different from the traditional calculation method. The result shows that the calculation method in this article can be used as an accurate method for evaluating performance indicators of AC digital bridges, which can better guide users to correctly use the technical indicators of each measurement point. At the same time, it can also avoid the misjudgment of measurement personnel and has a higher practicability.